The present invention relates to a polymerisation process.
The production of polymer powder by polymerisation reactions of monomers in the presence of catalysts is well-known. For example, processes are known and widely operated commercially using both fluidised bed reactors and slurry phase reactors.
In the gas fluidised bed polymerisation of olefins the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid.
In a slurry polymerisation process the polymerisation is conducted in a stirred tank or, preferably, a continuous loop reactor comprising mainly polyolefin, hydrocarbon diluent and a catalyst for the polymerisation. Polymer product is removed from the reactor in the form of a slurry in the diluent.
Inert gases, and in particular nitrogen, can be used or provided for a number of purposes in the overall polymerisation process. Some of these uses occur during normal operation, whereas others are used during start-ups or shut-downs, or during process upsets.
For example, a potentially large requirement for inert gas can be found in downstream processing. In particular, the polymer product removed from the reactor in a polymerisation process may contain unreacted monomers and other hydrocarbon species (for example, hydrogen, methane, ethane, propane, butane, pentane, hexane) and these monomers and other hydrocarbons should be removed from the polymer product since failure to do so may lead to (a) hydrocarbons levels rising to explosive levels in downstream equipment or (b) environmental constraints being exceeded or (c) unacceptable product quality e.g. odours.
The removal of monomer and other residual hydrocarbons, which may be in gaseous or liquid form, is generally referred to as “degassing”. One method that may be used is to contact the produced polymer with a gas in a purge vessel, usually a counter-currently flowing gas. The gas can be an inert gas, in particular nitrogen, although it is also known to use reaction gases, and is generally referred to as “purging”.
Further, the polymer, even after degassing, can still contain residual levels of hydrocarbons, and for this reason downstream storage silos are also often provided with a purge using an inert gas to prevent build-up of such hydrocarbons in the silos.
A further example of a requirement for inert gas can be found in purging of feed treatment vessels. In particular, feeds to the process, such as fresh monomers, are generally passed through one or more treatment vessels comprising treatment beds to remove undesired components which might otherwise adversely influence the polymerisation process, for example by acting to poison the polymerisation catalyst. The treatment beds must be periodically treated to remove the undesired components, a process known as regeneration. Regeneration generally involves stopping passage of the fluid to be treated e.g. monomer, to the treatment vessel. The treatment bed is then usually depressurised and purged with an inert gas flow, generally heated, to remove the undesired components. Such processes are described generally in WO 2010/123748.
As further examples, inert gases may be used to purge valves, seals and filters to keep them free of fouling.
Inert gases, such as nitrogen even if not normally used, are also often provided as a back-up in case of a process upset. For example, in the event of a process upset nitrogen may be used to flush the reactor or other parts of the apparatus of reactive gases. This can include use of nitrogen in purge vessels where nitrogen is not otherwise typically used as the purge gas.
It can be seen from the above that inert gases may be used for a number of different steps in a polymerisation process, in particular in the downstream processing.
As a result of all of the above uses and, in particular the large number of potential uses of inert gas in the event of process upsets, it is necessary to provide a large inert gas capacity for the event it is required, but which inert gas capacity is normally not used. This can result in a large cost to the build and operation of a polymerisation plant, a cost which increases as the plant capacity increases.